Various commercial, scientific and military operations require that a platform on or in the ocean be accurately located with respect to an array of previously positioned transponders. The platform can be a vessel, a surface ship or a submersible.
Such technology is well known. The first step is to deploy an array of transponders underwater and thence to accurately calibrate their relative or absolute positions. Techniques for calibrating an array of underwater transponders are well known and are described in the applicant's prior U.S. Pat. No. 4,097,837, for example, and in other publications such as T. D. Henry's paper entitled "ACOUSTIC TRANSPONDER NAVIGATION", CONFERENCE: IEEE 1978, Position Location and Symposium, San Diego, CA (6-9 Nov. 1978).
After the array of transponders has been position calibrated, it is well known to use various methods of acoustically signaling between the array and a vessel, for example, so as to locate the vessel vis-a-vis the array. The array can comprise as few as two transponders, such as disclosed in U.S. Pat. No. 4,229,809 to Schwalbe. Schwalbe describes an acoustic underwater position measurement system for determining the location of objects, such as submersible vessels, mining apparatus, and the like with respect to a surface vessel.
U.S. Pat. No. 3,860,900 to Scudder teaches a method of monitoring the position of a towed underwater apparatus using an array of three submerged transponders.
Other U.S. patents dealing with techniques for locating something vis-a-vis an array of underwater transponders include U.S. Pat. Nos. 4,635,236; 4,555,779; 4,110,726. A paper by L. W. Proctor, "SONAR SYSTEM FOR FISH DETECTION IN DEEP WATER": Conference: Instrumentation on Oceanography, Bangor, Wales, (22-25 Sept. 1975), discusses the problems inherent in a hull mounted sonar system.
An advantage of using acoustic transponders is that they can provide a highly accurate navigational fix, but a concomitant disadvantage is that they can generally be used only over a relatively small area. Depending upon local bathymetry and weather conditions, acoustic transponders are generally spaced within a small multiple of the water depth in which they are located. If the object whose location is to be measured is either a ship or a submersible, on board ship noise from propulsion equipment, auxiliary equipment, propeller and water noise tend to mask acoustic signals coming from transponders and thereby impose further constraints on the distances over which this technology can be used. That is, the ship or submersible must be sufficiently close to the transponders so that the acoustic signals generated by the transponder can be heard over the noise generated by the ship or submersible. It is, therefore, an object of this invention to eliminate or reduce the masking of acoustic signals which can otherwise occur on a relatively noisy platform.